A method of scrolling an image (image-scrolling) at high speed is disclosed in JP-A-H11-161158. In this method, resolution of a displayed image at scrolling is decreased to one half to realize high-speed scrolling. Namely, when original map data shown in the upper portion of FIG. 13 are used for image-scrolling, scrolling map data are generated by thinning the original map data vertically and horizontally every other line as shown in the lower portion of FIG. 13. While the resolution of the scrolling map data is a half of the resolution of the original map data, a data amount of the scrolling map data is one fourth of that of the original map data. The generated scrolling map data are stored in a VRAM.
From the stored scrolling map data, each portion corresponding to a map image necessary for the image-scrolling is sequentially retrieved and displayed on a display. This results in realizing the image-scrolling at a quadruple speed of a speed at which images corresponding to the original map data are scrolled.
In recent years, a high-definition map image becomes available, and a data amount of the map image has been thereby increasing. If the high-definition map image is used for image-scrolling, the data amount of the map image is required to be much more thinned. In this case, the resolution of a scrolling image must be significantly decreased, so that the scrolling image becomes much less recognizable.
The above problem is remarkably exhibited in a car navigation system that has an image-scrolling function. The car navigation system typically has a low memory amount of VRAM due to cost restriction, while the system is required to display high-speed and recognizable scrolling image.